The moose population on a small island is observed to be given by the function
$$P(t) = 120 t - 0.4 t^4 + 900$$
where $t$ is the time (in months) since observations of the island began.

(a) The maximum population attained at

$t =$ months.

The maximum population is

$P(t) =$ moose.

(b) When does the moose population disappear from the island?

$t =$ months